1. Field of the Invention
The present invention relates to a device for controlling a driving voltage for driving a load such as a liquid crystal display panel by an AC driving method, and more particularly to a device capable of quickly increasing/decreasing a voltage value of a driving voltage.
2. Description of the Background Art
In order to drive a liquid crystal display panel of a portable device (e.g., a mobile telephone) by an AC driving method (e.g., line inversion driving method), a conventional liquid crystal display driving device includes a driving voltage control device for controlling a driving voltage supplied to the counter electrode of the liquid crystal display panel. The driving voltage control device inverts the polarity of the driving voltage according to a predetermined timing.
FIG. 16 shows a general configuration of a conventional driving voltage control device 9. The device 9 includes a timing control section 91, a VCOM voltage generation section 92, a VCOMH operational amplifier 93H, a VCOML operational amplifier 93L, smoothing capacitors C94H and C94L, switches SW1 and SW2 and an output terminal 95. The device 9 alternately outputs driving voltages VCOMH and VCOML to the counter electrode (not shown) of the liquid crystal display panel.
The timing control section 91 outputs the control signals Sa and Sb. The control signal Sa indicates the voltage value of the driving voltage VCOMH to be generated by the VCOM voltage generation section 92. The control signal Sb indicates the voltage value of the driving voltage VCOML to be generated by the VCOM voltage generation section 92. The timing control section 91 receives a timing signal TIMING, and outputs control signals S1 and S2. The timing signal TIMING indicates the timing, according to which the voltage levels of the control signals S1 and S2 are switched from “H level” to “L level” (or from “L level” to “H level”).
The VCOM voltage generation section 92 is configured so as to generate the driving voltages VCOMH and VCOML according to the control signals Sa and Sb output from the timing control section 91. The VCOM voltage generation section 92 may be, for example, an RDAC (Resistance Digital Analog Converter), and has a configuration as shown in FIG. 2.
The switch SW1 is connected between a node N94H and the output terminal 95. The switch SW2 is connected between a node N94L and the output terminal 95. The switches SW1 and SW2 are on when the control signals S1 and S2, respectively, from the timing control section 91 are at “H level”, and off when they are at “L level”.
FIG. 16 shows a panel load C(LC) as the load capacitor of the liquid crystal display panel.
Internal Configuration of VCOMH Operational Amplifier 93H
FIG. 17 shows an internal configuration of the VCOMH operational amplifier 93H shown in FIG. 16. The VCOMH operational amplifier 93H includes input transistors TA1-H to TA5-H, output transistors TB1-H and TB2-H and a phase compensation capacitor CB-H. The input transistors TA1-H to TA5-H together form a differential stage 93AH of the VCOMH operational amplifier 93H. The output transistors TB1-H and TB2-H and the phase compensation capacitor CB-H together form an output stage 93BH of the VCOMH operational amplifier 93H.
Internal Configuration of VCOML Operational Amplifier 93L
FIG. 18 shows an internal configuration of the VCOML operational amplifier 93L shown in FIG. 16. The VCOML operational amplifier 93L includes input transistors TA1-L to TA5-L, output transistors TB1-L and TB2-L and a phase compensation capacitor CB-L. The input transistors TA1-L to TA5-L together form a differential stage 93AL of the VCOML operational amplifier 93L. The output transistors TB1-L and TB2-L and the phase compensation capacitor CB-L together form an output stage 93BL of the VCOML operational amplifier 93L.
Operation
Next, an operation of the driving voltage control device 9 shown in FIG. 16 will be described with reference to FIG. 19. In the illustrated example, the voltage value of the driving voltage VCOMH is “+3 V” and the voltage value of the driving voltage VCOML is “−3 V”.
In the period t0-t1, the timing control section 91 keeps the control signal S1 at “L level” and the control signal S2 at “H level”. A voltage V95 at the output terminal 95 is “−3 V”.
At time t1, the timing control section 91 brings the control signal S1 to “H level” and the control signal S2 to “L level” according to the timing signal TIMING. Thus, the switch SW1 is turned on, and the output terminal 95 is connected to the VCOMH operational amplifier 93H. Since the potential V95 at the output terminal 95 (the potential of the panel load C(LC)) is “−3 V”, a current flows from a VCOMH operational amplifier 13H to the output terminal 95 (the panel load C(LC)) until the potential V95 at the output terminal 95 reaches “+3 V” (until the rising time tpH elapses).
At time t3, the timing control section 91 brings the control signal S1 to “L level” and the control signal S2 to “H level” according to the timing signal TIMING from an external component. Thus, the switch SW2 is turned on, and the output terminal 95 is connected to the VCOML operational amplifier 93L. Since a potential V95 at the output terminal 95 is “+3 V”, a current flows from the output terminal 95 to a VCOML operational amplifier 93L until the potential V95 at the output terminal 95 reaches “−3 V” (until the falling time tpL elapses).
In the period t4-t9, an operation similar to that in the period t0-t4 is performed.
Thus, when inverting the polarity of the driving voltage, the panel load C(LC) needs to be charged/discharged, whereby the potential V95 at the output terminal 95 slowly increases (or decreases).
Moreover, with the recent increase in the resolution of a liquid crystal display panel, the capacitance value of the panel load C(LC) is increasing. Furthermore, with an increasing demand for a mobile telephone capable of displaying a motion picture, the panel load C(LC) needs to be charged/discharged more quickly. In order to quickly charge/discharge the panel load C(LC) having a large capacitance value (i.e., to shorten the rising time tpH and the falling time tpL), it is necessary to apply a high voltage to an operational amplifier included in the driving voltage control device. In view of this, a transistor with a high breakdown voltage is used as the VCOMH operational amplifier 93H and the VCOML operational amplifier 93L shown in FIG. 16.
Driving voltage control devices in which the bias current of an operational amplifier is controlled so as to reduce the power consumption while the circuit area is reduced so as to prevent an increase in cost are known in the art (see, for example, Japanese Laid-Open Patent Publication No. 2003-216256).